President Barack Obama has committed to preventing Iran from getting a nuclear weapon, by military force if necessary. Last year, he told Jeffrey Goldberg of the Atlantic, “As President of the United States, I don’t bluff.” But why should anyone believe him when North Korea has gone nuclear with impunity? It’s an uncomfortable question for the Administration, at a particularly bad moment.

North Korea’s third nuclear test, confirmed overnight by its KCNA news agency, comes just as the U.S. is entering a new round of diplomacy with Iran. Talks are scheduled between Iran and the International Atomic Energy Agency this week; negotiations between Iran, the U.S. and other world powers will take place on Feb. 26 in Kazakhstan.

Expectations are low for both talks, but at the least the Administration is hoping to push back further into the future any possible military action against the Iranian nuclear program. A harder line by Tehran in the wake of the North Korean test could move things in the wrong direction.

But current and former Administration officials argue the two situations are different and that Iran would be making a mistake to see strength in North Korea’s defiance of international sanctions and its abandonment of the Nuclear Non-Proliferation Treaty. Administration officials admit that they have adopted a policy of containing a nuclear North Korea, even as they say Pyongyang’s program is “unacceptable,” but they say there’s no way they would cave in to containing Iran if Tehran went nuclear. They explain their position like this:

First, containing a nuclear North Korea, as the U.S. contained Russia in the Cold War, is possible. Containing a nuclear Iran is not. Japan and South Korea accept the U.S. nuclear umbrella to protect them, thereby preventing a regional arms race that could lead to nukes all over Asia. In the Middle East, Saudi Arabia, Egypt and Turkey are unlikely to accept U.S. guarantees of protection against a nuclear Iran, so would pursue their own programs.

That would mean a region that is already prone to conflict suddenly awash in nukes. “The risk of having a crisis [in a nuclear Middle East] that moves very quickly, and not controllably, increases geometrically the prospect of a nuclear war,” says a former senior Administration official familiar with Obama’s thinking on Iran.

Second, say former and current Administration officials, the North Korea situation was inherited: North Korea kicked out nuclear watchdogs and tested its first nuclear weapon before Obama came into office, so Pyongyang was already nuclear and was already being contained. Rather than preventing the North from going nuclear, Obama would have had to disarm the country. “Then you’re talking about a rollback strategy,” says another former senior Administration official, which is harder than hitting a program like Iran’s before it has a nuclear weapon stashed away for its defense.

Third, the Administration says, even if it wanted to take out the North Korean nuclear sites, it can’t: it doesn’t know where many of them are. “Nobody has any idea where the North Korean stuff is,” says another former senior Administration official. “There are facilities that can be hit, but in terms of the reprocessed plutonium and aspects of the uranium-enrichment program, we have much less of an idea,” compared with Iran.

Last, the regional politics are different. In the Middle East, Israel has made it clear publicly and privately that it will take military action to stop Iran from going nuclear. If it did, the U.S. likely would be drawn into a war regardless. In Asia, a U.S. attack on North Korea, nominally an ally of China, would be more destabilizing than the current approach of trying to contain Pyongyang, the Administration argues.

Ultimately, the U.S. hopes China will pressure North Korea to stop the pursuit of nuclear weapons. China’s response to North Korea’s test was firm, but noncommittal. The Administration is pursuing further sanctions against the North at the U.N. today, and is looking for Chinese support.